Semiconductor light emitting device

ABSTRACT

According to one embodiment, a semiconductor light emitting device includes an n-type semiconductor layer, a p-type semiconductor layer, and a light emitting part. The n-type semiconductor layer includes a nitride semiconductor. The p-type semiconductor layer includes a nitride semiconductor. The light emitting part is provided between the n-type and the p-type semiconductor layers and includes an n-side barrier layer and a first light emitting layer. The first light emitting layer includes a first barrier layer, a first well layer, and a first AlGaN layer. The first barrier layer is provided between the n-side barrier layer and the p-type semiconductor layer. The first well layer contacts the n-side barrier layer between the n-side and the first barrier layer. The first AlGaN layer is provided between the first well layer and the first barrier layer. A peak wavelength λp of light emitted from the light emitting part is longer than 515 nanometers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2011-39281, filed on Feb. 25,2011; the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a semiconductor lightemitting device.

BACKGROUND

A light emitting diode (LED) which is a semiconductor light emittingdevice using a nitride semiconductor is used, for example, for a displayapparatus, lighting apparatus or the like. A laser diode (LD) is usedfor a light source for reading or writing from or onto a high-densitymemory disc or the like.

For such semiconductor light emitting device, a higher luminance isdemanded.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and FIG. 1B are schematic cross-sectional views illustrating theconfiguration of a semiconductor light emitting device according to anembodiment;

FIG. 2 is a schematic cross-sectional view illustrating theconfiguration of a semiconductor light emitting device according to theembodiment;

FIG. 3 is a schematic cross-sectional view illustrating theconfiguration of a part of the semiconductor light emitting deviceaccording to the embodiment;

FIG. 4 is a graph illustrating characteristics of the semiconductorlight emitting devices;

FIG. 5 is a transmission electron microscope image illustrating theconfiguration of the semiconductor light emitting device according tothe embodiment;

FIG. 6 is a schematic cross-sectional view illustrating theconfiguration of a semiconductor light emitting device according to theembodiment; and

FIG. 7 is a graph illustrating an example of characteristics of thesemiconductor light emitting devices.

DETAILED DESCRIPTION

In general, according to one embodiment, a semiconductor light emittingdevice includes an n-type semiconductor layer, a p-type semiconductorlayer, and a light emitting part. The n-type semiconductor layerincludes a nitride semiconductor. The p-type semiconductor layerincludes a nitride semiconductor. The light emitting part is providedbetween the n-type semiconductor layer and the p-type semiconductorlayer and includes an n-side barrier layer and a first light emittinglayer.

The first light emitting layer includes a first barrier layer, a firstwell layer, and a first AlGaN layer. The first barrier layer is providedbetween the n-side barrier layer and the p-type semiconductor layer. Thefirst well layer contacts the n-side barrier layer between the n-sidebarrier layer and the first barrier layer. The first AlGaN layer isprovided between the first well layer and the first barrier layer. Thefirst AlGaN layer has a layered-form and includes Al_(z1)Ga_(1-z1)N(0.25<z1≦1). A peak wavelength λp of light emitted from the lightemitting part is longer than 515 nanometers.

In general, according to another embodiment, a semiconductor lightemitting device includes an n-type semiconductor layer, a p-typesemiconductor layer, and a light emitting part. The n-type semiconductorlayer includes a nitride semiconductor. The p-type semiconductor layerincludes a nitride semiconductor. The light emitting part is providedbetween the n-type semiconductor layer and the p-type semiconductorlayer and includes an n-side barrier layer and a first light emittinglayer. The first light emitting layer includes a first barrier layer, afirst well layer, and a first AlGaN layer. The first barrier layer isprovided between the n-side barrier layer and the p-type semiconductorlayer. The first well layer contacts the n-side barrier layer betweenthe n-side barrier layer and the first barrier layer. The first AlGaNlayer is provided between the first well layer and the first barrierlayer. The first AlGaN layer has a layered-form and includesAl_(z1)Ga_(1-z1)N (0<z1≦1). An Al composition ratio z1 in group III ofthe first AlGaN layer and a peak wavelength λp (nanometer) of lightemitted from the light emitting part satisfies a condition of1.15z1>0.0024λp−0.972>0.90z1.

Various embodiments will be described hereinafter with reference to theaccompanying drawings.

The drawings are schematic or conceptual; and the relationships betweenthe thicknesses and widths of portions, the proportions of sizes amongportions, etc., are not necessarily the same as the actual valuesthereof. Further, the dimensions and the proportions may be illustrateddifferently among the drawings, even for identical portions.

In the specification and the drawings of the application, componentssimilar to those described in regard to a drawing thereinabove aremarked with like reference numerals, and a detailed description isomitted as appropriate.

(Embodiment)

FIG. 1A and FIG. 1B are schematic cross-sectional views illustrating theconfigurations of semiconductor light emitting devices according to anembodiment.

As shown in FIG. 1A, a semiconductor light emitting device 110 accordingto the embodiment includes an n-type semiconductor layer 20, a p-typesemiconductor layer 50 and a light emitting part 40.

The n-type semiconductor layer 20 and the p-type semiconductor layer 50respectively include nitride semiconductors.

The light emitting part 40 is provided between the n-type semiconductorlayer 20 and the p-type semiconductor layer 50. The light emitting part40 includes an n-side barrier layer BLN and a first light emitting layerEL1. This first light emitting layer EL1 is provided between the n-sidebarrier layer BLN and the p-type semiconductor layer 50.

The first light emitting layer EL1 includes a first barrier layer BL1, afirst well layer WL1 and a first AlGaN layer ML1.

The first barrier layer BL1 is provided between the n-side barrier layerBLN and the p-type semiconductor layer 50. The first well layer WL1contacts the n-side barrier layer BLN between the n-side barrier layerBLN and the first barrier layer BL1. The first AlGaN layer ML1 isprovided between the first well layer WL1 and the first barrier layerBL1, and includes Al_(z1)Ga_(1-z1)N (0.25<z1≦1). This first AlGaN layerML1 is formed in a layer.

In the semiconductor light emitting device 110, a single well layer WLis provided. As such, the light emitting part 40 may have a singlequantum well (SQW: Single Quantum Well) structure.

As shown in FIG. 1B, in another semiconductor light emitting device 111according to the embodiment, the light emitting part 40 further includesa second light emitting layer EL2.

This second light emitting layer EL2 includes a second barrier layerBL2, a second well layer WL2 and a second AlGaN layer ML2.

The second barrier layer BL2 is provided between the first barrier layerBL1 and the p-type semiconductor layer 50. The second well layer WL2contacts the first barrier layer BL1 between the first barrier layer BL1and the second barrier layer BL2. The second AlGaN layer ML2 is providedbetween the second well layer WL2 and the second barrier layer BL2, andincludes Al_(z)2Ga_(1-z2)N (0.25<z2≦1). The second AlGaN layer ML2 isformed in a layer. In the semiconductor light emitting device 111, aplurality of well layers WL is provided. As such, the light emittingpart 40 may have a multiple quantum well (MQW: Multiple Quantum Well)structure. In the semiconductor light emitting device 111, the number ofthe well layers WL is four. Namely, the number of the light emittinglayers EL is four. However, the number of well layers WL is arbitrary inthe semiconductor light emitting device according to the embodiment.

The light emitting part 40 includes, for example, a plurality of lightemitting layers EL (the first light emitting layer EL1 to the n-th lightemitting layer ELn). Here, “n” is an integer of two or larger.

Here, it is assumed that the (i+1)-th light emitting layer EL(i+1) beprovided between the i-th light emitting layer ELi and the p-typesemiconductor layer 50. Here, “i” is an integer of one or larger.

The i-th light emitting layer ELi includes an i-th barrier layer BLi, ani-th well layer WLi and an i-th AlGaN layer MLi.

The (i+1)-th barrier layer BL(i+1) is provided between the i-th barrierlayer BLi and the p-type semiconductor layer 50. The (i+1)-th well layerWL(i+1) contacts the i-th barrier layer BLi between the i-th barrierlayer BLi and the (i+1)-th barrier layer BL(i+1). The (i+1)-th AlGaNlayer ML(i+1) is provided between the (i+1)-th well layer WL(i+1) andthe (i+1)-th barrier layer BL(i+1), and includes Al_(zi)Ga_(1-zi)N(0.25<zi≦1). The (i+1)-th AlGaN layer ML(i+1) is formed in a layer.

Hereinafter, the first barrier layer BL1 to the n-th barrier layer BLnmay be generically referred to as barrier layers BL. The first welllayer WL1 to the n-th well layer WLn may be generically referred to aswell layers WL. The first AlGaN layer ML1 to the n-th AlGaN layer MLnmay be generically referred to as AlGaN layers ML.

The plurality of AlGaN layers may have the same Al composition ratio (Alcomposition ratio in group III) and may have mutually different Alcomposition ratios. In an arbitrary AlGaN layer, however, the Alcomposition ratio z is set to 0.25<z≦1. In the following, it is assumedthat the plurality of AlGaN layers have the same Al composition ratio(constant Al composition ratio z) to simplify the explanations.

A bandgap energy of the i-th well layer WLi is smaller than that of thei-th barrier layer BLi, and is smaller than that of the n-side barrierlayer BLN.

For the well layer WL, for example, an InGaN layer is used, and for thebarrier layer BL, for example, the GaN layer is used. When the InGaNlayer is used for the barrier layer BL, the In composition ratio (Incomposition ratio in group III) in the barrier layer BL is lower thanthat in the well layer WL.

The thickness of the well layer WL is, for example, in a range of notless than 1.0 nanometer (nm) and not more than 5.0 nm. When thethickness of well layer WL is smaller than 1.0 nm, it is difficult toachieve emission of light having a longer wavelength than 515 nm. On theother hand, when the thickness of well layer WL is larger than 5.0 nm,degradation of the crystalline quality tends to occur. Furthermore, aspatial separation between the wavefunction of an electron and thewavefunction of a hole increases, and the light emission intensity tendsto be lowered.

The thickness of the barrier layer BL is, for example, in a range of notless than 3 nm and not more than 50 nm. When the thickness of thebarrier layer BL is smaller than 3 nm, a space between the well layersWL becomes narrower, which in turn tends to cause wavefunctioninterference between different well layers WL, or an alleviation ofstrain in these well layers WL. On the other hand, when the thickness ofthe barrier layer BL is larger than 50 nm, the light emitting layer ELbecomes too thick, and thereby an operating voltage increases.

FIG. 2 is a schematic cross-sectional view illustrating theconfiguration of a semiconductor light emitting device according to theembodiment. As shown in FIG. 2, the semiconductor light emitting device110(111) according to the embodiment further includes a substrate 10 anda buffer layer 11. The substrate 10 and the buffer layer 11 are providedas necessary, and may be omitted appropriately. The n-type semiconductorlayer 20 is provided between the substrate 10 and the light emittingpart 40. The buffer layer 11 is provided between the substrate 10 andthe n-type semiconductor layer 20.

For the substrate 10, for example, sapphire may be used. For thesubstrate 10, for example, sapphire (0001) substrate may be used.Furthermore, SiC substrate, Si substrate or GaN substrate may be usedfor the substrate 10. For the buffer layer 11, for example, a GaN layermay be used. On the buffer layer 11, the n-type semiconductor layer 20,the light emitting part 40 and the p-type semiconductor layer 50 areformed sequentially. After the above-mentioned semiconductor layer isformed on the buffer layer 11, the substrate 10 and the buffer layer 11may be removed.

A stacked body 10 s which includes the n-type semiconductor layer 20,the light emitting part 40 and the p-type semiconductor layer 50 has afirst major surface 10 a and a second major surface 10 b. The firstmajor surface 10 a is formed on the side of the p-type semiconductorlayer 50, and the second major surface 10 b is formed on the side of then-type semiconductor layer 20.

In this example, a part of the n-type semiconductor layer 20 faces thep-type semiconductor layer 50. On the side of first major surface 10 a,a p-side electrode 80 is provided in contact with the p-typesemiconductor layer 50. Further, on the side of the first major surface10 a, an n-side electrode 70 is provided in contact with the n-typesemiconductor. Here, as mentioned above, the substrate 10 (and thebuffer layer 11) may be removed, and the n-side electrode 70 may contactthe n-type semiconductor layer 20 on the side of the second majorsurface 10 b.

Here, the direction going from the n-type semiconductor layer 20 towardthe p-type semiconductor layer 50 is assumed to be a Z-axis direction.An axis perpendicular to this Z-axis is assumed to be an X-axis. An axisperpendicular to both the Z-axis and the X-axis is assumed to be aY-axis.

The n-type semiconductor layer 20 includes, for example, an n-sidecontact layer. For this n-side contact layer, the GaN layer containingn-type impurities is used. For the n-type impurities, for example, Si(silicon) is used.

The p-type semiconductor layer 50 may include, for example, a firstp-type layer 51, a second p-type layer 52 and a third p-type layer 53.The second p-type layer 52 is provided between the first p-type layer 51and the light emitting part 40. The third p-type layer 53 is providedbetween the second p-type layer 52 and the light emitting part 40. Forthe third p-type layer 53, for example, a p-type AlGaN layer is used.This third p-type layer 53 can serve, for example, as an electronoverflow blocking layer (current overflow blocking layer). The thirdp-type layer 53 may be omitted. For the second p-type layer 52, anMg-dopted p-type GaN layer is used. For the first p-type layer 51, ahigh concentration Mg-doped p-type GaN layer may be used. This p-typelayer 51 serves as a contact layer. For the p-type impurities, forexample, Mg (magnesium) may be used.

The major surface of the n-type semiconductor layer 20 is a c-face. Themajor surface of the p-type semiconductor layer 50 is also a c-face.However, for example, the major surface of the n-type semiconductorlayer 20 and the major surface of the p-type semiconductor layer 50 maybe slightly inclined from the c-face.

For the n-side electrode 70, for example, a composite film oftitanium-platinum-gold (Ti/Pt/Au) is used.

For the p-side electrode 80, for example, indium tin oxide (ITO) or thelike may be used. For the p-side electrode 80, a composite film ofnickel-gold (Ni/Au) or the like may be used.

The semiconductor light emitting device according to the embodiment mayfurther includes a multilayer stacked body provided between the n-typesemiconductor layer 20 and the light emitting part 40.

FIG. 3 is a schematic cross-sectional view illustrating theconfiguration of a part of the semiconductor light emitting deviceaccording to the embodiment. Specifically, FIG. 3 illustrates anexamplary configuration of a multilayer stacked body 30. This multilayerstacked body 30 is provided between the n-type semiconductor layer 20and the light emitting part 40.

The multilayer stacked body 30 includes a plurality of thick film layersSA and a plurality of thin film layers SB, which are alternately stackedalong the +Z direction. Here, the thickness of the thin film layer SB isequal to or smaller than that of the thick film layer SA. The thin filmlayer SB has a composition different from that of the thick film layerSA.

The plurality of thin film layers SB, for example, includes a first thinfilm layer SB1 to an m-th thin film layer SBm. Here, “m” is an integerof two or larger. The plurality of thick film layers SA includes a firstthick film layer SA1 to an m-th thick film layer SAm. The plurality ofthick film layers SA may further include an (m+1)-th thick film layerSA(m+1). The multilayer stacked body 30 may have, for example, asuperlattice structure.

The thickness of the thick film layer SA is, for example, in a range ofnot less than 1 nm and not more than 3 nm. The thickness of the thinfilm layer SB is, for example, less than 1.5 nm, and is not more thanthe thickness of thick film layer SA.

The multilayer stacked body 30 may include a nitride semiconductor.

Specifically, the thick film layer SA includes, for example, GaN. Thethin film layer SB includes InGaN. For the thick film layer SA, forexample, the GaN layer substantially not including In is used. In thecase where the multilayer stacked body 30 includes an InGaN layer, it ispreferable that the In composition be lower than that in the well layerWL which will grow later. The multilayer stacked body 30 is provided asnecessary, and may be omitted appropriately. In the following,explanations will be given through the case where the multilayer stackedbody 30 is provided.

In the semiconductor light emitting device 110(111) according to theembodiment, the peak wavelength λp of the light emitted from the lightemitting part 40 is longer than 515 nm. Namely, when the peak wavelengthλp exceeds 515 nm, an AlGaN layer which has a higher Al compositionratio z than 0.25 is provided between the well layer WL and the barrierlayer BL formed on the side of the p-type semiconductor layer 50 whenseen from the well layer WL. As a result, a high luminance can beachieved.

This characteristic was found by the following experiments. In thefollowing, an original experiment performed by inventors will beexplained.

For the following method of growing semiconductor layers, the MOVPE(Metal-Organic Vapor Phase Epitaxy) was used.

First, a thermal cleaning of the substrate 10 of sapphire (0001) wasperformed at a susceptor temperature of 1100° C.

Next, the susceptor temperature was decreased to 500° C., and the bufferlayer 11 (GaN layer) was grown on the major surface of the substrate 10.Next, after the susceptor temperature was increased to 1120° C., aSi-doped n-type GaN layer was grown as the n-type semiconductor layer20.

Thereafter, the multilayer stacked body 30 was formed. Specifically,carrier gas was changed from H₂ gas into N₂ gas, and the susceptortemperature was decreased to 850° C. Then, layers of 20 periods, eachperiod having an In_(0.08)Ga_(0.92)N layer with a thickness of 1 nm(thin film layer SB) and a GaN layer with a thickness of 3 nm (thickfilm layer SA), were formed. As a result, the multilayer stacked body 30was formed. Here, at least any of the thin film layer SB and the thickfilm layer SA may contain Si. This multilayer stacked body 30 mayinclude a function as the n-type semiconductor layer 20.

Then, the growth was interrupted, and the susceptor temperature wasincreased to 950° C. At this temperature, the n-side barrier layer BLNwas grown. In this experiment, the Si-doped GaN layer was grown as then-side barrier layer BLN. The thickness of the n-side barrier layer BLNis 12.5 nm. Here, the n-side barrier layer BLN may not be doped with Si.

Thereafter, the susceptor temperature was decreased to temperatures in arange of not less than 700° C. and not more than 800° C., and the InGaNlayer was grown as the first well layer WL1. The first well layer WL1has a thickness around 3 nm, and an In composition ratio of 0.23.

Thereafter, continuously, the Al_(0.30)Ga_(0.70)N layer with a thicknessof 1.5 nm was grown as the first AlGaN layer ML1, and further the GaNlayer with a thickness of 0.5 nm to become a first cap layer was grownthereon.

Namely, the first AlGaN layer ML1 and the first cap layer are grownsubstantially at the same temperature as the growth temperature of thefirst well layer WL. As a result, the first well layer WL1 and the firstAlGaN layer ML1 can be maintained flat.

Next, the susceptor temperature was increased to 850° C., and the GaNlayer to become the first barrier layer BL1 was grown. The thickness ofthis GaN layer is 12.5 nm.

As a result, the first light emitting layer EL1 was formed.Subsequently, the second light emitting layer EL2 to the fourth lightemitting layer EL4 were formed in the same manner as the above.

Thereafter, an Mg-doped AlGaN layer was formed as the third p-type layer53. An Mg-doped GaN layer was then formed as the second p-type layer 52.Subsequently, a high concentration Mg-doped GaN layer was formed as thethird p-type layer 53. As a result, the p-type semiconductor layer 50was formed.

Then, the sample was taken out of the reaction chamber, and thesemiconductor light emitting device 111 was formed after having gonethrough the etching and the electrode forming processes and the like.

In the above, a plurality of semiconductor light emitting devices 111can be obtained from a single wafer (substrate 10). In this experiment,when the peak wavelength λp of light emitted from a plurality ofsemiconductor light emitting devices 111 was measured, peak wavelengthsλp were different among the plurality of semiconductor light emittingdevices 111. These variations in peak wavelengths λp were based on, forexample, variations of 1 n composition ratio z in the wafer surface ofthe well layers WL, and variations in thickness in the wafer surface ofthe well layers WL and the like. As a result, the plural semiconductorlight emitting devices 111, having different peak wavelengths λp, wereobtained.

Samples in which the Al composition ratio z in the AlGaN layer ML wasvaried were also prepared using the above process. Samples having the Alcomposition ratios z in the AlGaN layer ML of 0.09, 0.14 and 0.18 wereprepared as a semiconductor light emitting device 119 a according to thefirst reference example, a semiconductor light emitting device 119 baccording to the second reference example and a semiconductor lightemitting device 119 c according to the third reference example,respectively.

Further, a sample without having formed therein the AlGaN layer ML (thesemiconductor light emitting device 119 d according to the fourthreference example) was also prepared. In this semiconductor lightemitting device 119 d, the cap layer was formed continuously after thewell layer WL was formed. The temperature was then increased asexplained above to grow the barrier layer BL. Thereafter, the foregoingprocess was repeated, thereby forming the light emitting part 40. Theprocesses other than the above were the same as those for thepreparation of the semiconductor light emitting device 111.

For the semiconductor light emitting devices 119 a to 119 d of the firstthrough fourth reference examples, a plurality of semiconductor lightemitting devices having mutually different peak wavelengths λp were alsoobtained from the respective wafers.

FIG. 4 is a graph illustrating characteristics of the semiconductorlight emitting device. Namely, FIG. 4 show results of measurements onthe characteristics of the semiconductor light emitting devices 119 a to119 d according to the first through fourth reference examples. In thegraph, the horizontal axis corresponds to the peak wavelength λp in thesample of each semiconductor light emitting device. The vertical axiscorresponds to an output power OP (in logarithmic expression). Here, theoutput power OP indicates a value when the forward current was 20 mA.

As shown in FIG. 4, in each of the semiconductor light emitting device111 and the semiconductor light emitting devices 119 a to 119 daccording to the first through fourth reference examples, output powersOP were lowered as the peak wavelength λp becomes longer.

Particularly, in the semiconductor light emitting device 119 d accordingto the fourth reference example without having the AlGaN layer ML, theoutput power OP was lowered significantly when the peak wavelength λpbecame longer than 500 nm.

In the semiconductor light emitting devices 119 a to 119 c according tothe first to third reference examples, although a decrease in outputpower OP in the region of a longer wavelength than 500 nm was improvedas compared to the case of the semiconductor light emitting device 119d, the improvements were insufficient.

In contrast, in the semiconductor light emitting device 111 according tothe embodiment, a decrease in output power OP in the region of a longerwavelength than 500 nm was remarkably improved.

On the other hand, in the semiconductor light emitting device 111, inthe region of a shorter wavelength than 500 nm, an output power OP wasat the same level as or even lower than those of the reference examples.

As described, under the conditions that the AlGaN layer ML is used, andthe Al composition ratio z is as high as 0.3, an output power OP in theregion of a longer wavelength than 500 nm is improved as compared to thereference examples.

From the results shown in FIG. 4, according to the semiconductor lightemitting device 111 of the embodiment, when the Al composition ratio zis set to 0.3 or higher, the output power OP is reliably improved fromthe reference examples in the region of a longer wavelength than 515 nm.

Additionally, although the above experiment was performed using theAlGaN layer ML with the Al composition ratio z of 0.3, taking intoconsideration the difference from the characteristics of thesemiconductor light emitting device 119 c according to the thirdreference example with the Al composition ratio z of 0.18, it can berecognized that the output power OP is reliably improved in the regionof a longer wavelength than 515 nm under the condition that the Alcomposition ratio z of the AlGaN layer ML is higher than 0.25.

The configuration of the embodiment is constructed on the basis of theforegoing characteristics newly discovered from these experiments.

Namely, in the semiconductor light emitting device in which the peakwavelength λp of light emitted from the light emitting part 40 is longerthan 515 nm, the AlGaN layer ML including Al_(z)Ga_(1-z)N (0.25≦z≦1) isused. This AlGaN layer ML is provided between the well layer WL and thebarrier layer WL on the side of the p-type semiconductor layer 50 ofthat well layer WL. Namely, the first AlGaN layer ML1 is providedbetween the first well layer WL and the first barrier layer BL1. Withthis configuration, a high luminance can be achieved.

It is considered that the reason why such high luminance can be achievedfrom the configuration according to the embodiment is that a reductionin light emission efficiency can be suppressed due to thequantum-confined Stark effect.

For example, in the case of the semiconductor light emitting device 119d according to the fourth reference example, a strain is applied to thewell layer WL, which generates a piezoelectric field. Further, it isconsidered that with this piezoelectric field, an overlap integral valuebetween the wavefunction of a hole and the wavefunction of an electronis decreased, which in turn decreases the light emission efficiency.Namely, in the fourth reference example, for example, the wavefunctionof an electron of the well layer WL is protruded to the side of thep-type semiconductor layer 50. Particularly, in the well layer WL of alonger wavelength than 500 nm, the strain becomes larger, and thereforethe foregoing tendency becomes more outstanding.

In this state, by disposing the AlGaN layer ML on the side of the p-typesemiconductor layer 50 in the well layer WL, it is possible to suppressthe leakage of electrons to the side of the p-type semiconductor layer50. Specifically, an electron has a small effective mass. Thus, bydisposing the AlGaN layer ML, the wavefunction of an electron is shiftedmore to the side of n-type semiconductor layer 20 than the wavefunctionof a hole. As a result, it is possible to increase an overlap integralvalue between the wavefunction of a hole and the wavefunction of anelectron.

The foregoing effect can be appreciated more for the higher Alcomposition ratio z in the AlGaN layer ML. This is consistent with theresults shown in FIG. 4.

It can be seen from the experimental results shown in FIG. 4 thatimprovements in luminance can be fully appreciated when the Alcomposition ratio z in the AlGaN layer ML is higher than 0.25.

In the embodiment, however, when the Al composition ratio z in the AlGaNlayer ML is excessively high, crystalline quality may be adverselyaffected. Moreover, when the Al composition ratio z is excessively high,an effect of shifting the wavefunction of an electron is exhibitedexcessively, and an overlap integral value between the wavefunction ofan electron and the wavefunction of a hole tends to be decreased on thecontrary, resulting in a decrease in light emission efficiency. Asdescribed, there may exist an appropriate relationship between the peakwavelength λp and the Al composition ratio z in the AlGaN layer ML.Therefore, it is more preferable that the Al composition ratio z in theAlGaN layer ML be set in a range of more than 0.25 and not more than0.5. With this configuration, it is possible to realize both highluminance and desirable crystalline quality. As a result, asemiconductor light emitting device of high luminance and highefficiency can be obtained.

In the embodiment, the major surface of the substrate 10 is a c-face.With this configuration, a major surface of each crystalline layer (then-type semiconductor layer 20, the light emitting layer EL and thep-type semiconductor layer 50, and the like) is a c-face which is apolar face. In this case, the foregoing quantum-confined Stark effect bythe piezoelectric field tends to occur. In response, the embodimentadopts the AlGaN layer ML to suppress such effect.

Additionally, it may be arranged (as a reference example) such that theAlGaN layer is provided between the well layer WL and the barrier layerBL formed on the side of the n-type semiconductor layer 20 of that welllayer WL. However, with this configuration, the wavefunction of anelectron is shifted in a direction opposite to the above-explaineddirection. Thus, it is considered that the effect of suppressing areduction in light emission efficiency caused by the quantum-confinedStark effect cannot be expected.

Specifically, for example, in the first light emitting layer EL1,In_(x2)Ga_(1-x2)N (0≦x2<1) is used for the n-side barrier layer BLN incontact with the first well layer WL1. Moreover, In_(x1)Ga_(1-x1)N(0≦x1<1) is used for the barrier layer BL (the first barrier layer BL1and the like).

FIG. 5 is a transmission electron microscope image illustrating theconfiguration of the semiconductor light emitting device according tothe embodiment.

FIG. 5 is an image of a cross-section of a crystalline layer (lightemitting part 40) of the semiconductor light emitting device 111.

As shown in FIG. 5, in the semiconductor light emitting device 111, theAlGaN layer ML is formed in a layer. For example, neither the regionswhere through holes are formed nor the deeply recessed regions areformed in the AlGaN layer ML. For example, a part of the well layer WLis not substantially in contact with the barrier layer BL resulting froman exposed part of the well layer WL due to the recession or the throughholes of the AlGaN layer ML.

In FIG. 5, in each layer of the AlGaN layer ML, flatness at anatomic-level is observed. For example, an RMS value for the thickness ofthe AlGaN layer ML (the first AlGaN layer ML1 and the like) is 0.5 nm orsmaller.

There is an attempt to lower a threshold voltage or a drive voltage ofthe device by disposing an intermediate layer of AlGaN on the welllayer. In this case, the intermediate layer of AlGaN has a net structurein which a plurality of recessed regions and penetrated regions areformed in the surface thereof. The foregoing recessed regions andpenetrated regions occupy 10% or more of the surface of the intermediatelayer. It is considered that such net structure is formed in thefollowing manner. That is, the intermediate layer is formed at lowtemperatures, and then the temperature is raised to the growthtemperature of the barrier layer formed on the intermediate layer. Inthis state, the intermediate layer and the like are resolved, therebyforming the net structure.

In contrast, according to the embodiment, the cap layer is formed on theAlGaN layer ML as described above. Here, temperatures at which this caplayer is formed are around the same temperatures at which the AlGaNlayer ML is formed (the same temperatures in the case of the aboveexperiment). Then, after forming the cap layer, the temperature isincreased to high temperatures, at which the barrier layer BL is formed.By carrying out these processes, it is possible to suppress a part ofthe AlGaN layer ML from being resolved. Therefore, according to theembodiment, the layer form in the grown state can be maintained in theAlGaN layer ML. Namely, the AlGaN layer ML does not have a netstructure.

According to the embodiment, in the AlGaN layer ML, an area of therecessed regions or regions in which through holes are formed occupiesless than 10% of the area of the layer surface of the AlGaN layer ML.Namely, substantially no such region exists.

If the barrier layer BL is formed on the AlGaN layer ML at hightemperatures in the state where the cap layer is not formed, adeformation occurs in the AlGaN layer ML. Further, it is considered thatsuch deformation becomes outstanding particularly when the Alcomposition ratio z is higher than 0.25.

On the contrary, when the AlGaN layer ML is formed in a layer (flat) inthe AlGaN layer ML having a higher Al composition ratio z than 0.25, itcan be assumed that the barrier layer BL is formed at high temperaturesafter forming the foregoing cap layer.

FIG. 6 is a schematic cross-sectional view illustrating theconfiguration of a semiconductor light emitting device according to theembodiment.

As shown in FIG. 6, in the semiconductor light emitting device 112according to the embodiment, the cap layer is formed. For example, thefirst light emitting layer EL1 further includes the first cap layer CL(cap layer CL) which is in contact with both the first AlGaN layer ML1and the first barrier layer BL1 and which includes a nitridesemiconductor.

As a result, the layer form is maintained in the first AlGaN layer ML1.This cap layer CL may be or may not be observed, for example, by theanalytical method such as the transmission electron microscopyobservation or the like.

The semiconductor light emitting device 112 shown in FIG. 6 has a singlequantum well (SQW) structure. However, the embodiment is not limited tothis, and the cap layer CL may be formed in the semiconductor lightemitting device 111 of an MQW structure.

Specifically, it may be arranged such that in the case where a pluralityof light emitting layers EL is provided, the i-th light emitting layerELi may further include an i-th cap layer CL which is in contact withboth the i-th AlGaN layer MLi and the i-th barrier layer BLi and whichincludes a nitride semiconductor.

As a result, the AlGaN layer ML is formed in a layer. Namely, this AlGaNlayer ML does not have a net structure. For example, the RMS value forthe thickness of the AlGaN layer ML becomes 0.5 nm or smaller. In thisexample, the thickness of the AlGaN layer ML is around 1.5 nm. Thus,variations in thickness are plus or minus 33% or smaller (0.5 nm/1.5 nm)of an average thickness of the AlGaN layer ML.

In the embodiment, it is preferable that the growth temperatures of thebarrier layer BL be not lower than the growth temperatures of the welllayer WL, and that a difference between the growth temperatures of thebarrier layer BL and the growth temperatures of the well layer WL be200° C. or smaller. If the growth temperatures of the barrier layer BLare lower than the growth temperatures of the well layer WL, pits tendto generate in the barrier layer BL. Moreover, if the growthtemperatures of the barrier layer BL are higher than the growthtemperatures of the well layer WL, and the difference between the growthtemperatures of the barrier layer BL and the growth temperatures of thewell layer WL becomes larger than 200° C., the well layer WL tends todeteriorate.

In the embodiment, it is preferable that the thickness of the n-sidebarrier layer BLN be in a range of not less than 3 nm and not more than20 nm. When the thickness of the n-side barrier layer BLN is less than 3nm, a sufficient flatness of the surface cannot be obtained. On theother hand, when the thickness of the n-side barrier layer BLN is largerthan 50 nm, it is liable that the effect of alleviating the strain ofcrystals by the multilayer stacked body 30 becomes less effective andthe drive voltage becomes higher. Namely, when the thickness of then-side barrier layer BLN is set in a range of not less than 3 nm and notmore than 50 nm, it is possible to realize high flatness, highcrystalline quality and high light emission efficiency.

Additionally, in the process of forming the n-side barrier layer BLN, itis preferable that N₂ gas be used for the carrier gas when the susceptortemperature is in a range of not less than 600° C. and not more than1000° C. Here, H₂ gas may be further added. In this case however, anamount of flow of H₂ gas is set to or below a half of the total amountof gas flow. As a result, high crystalline quality can be achieved withease.

From the experimental results shown in FIG. 4, it is considered thatthere should be an appropriate relationship between the peak wavelengthλp and the Al composition ratio z in the AlGaN layer ML to realize ahigh luminance (output power OP). Regarding this, explanations will begiven below.

FIG. 7 is a graph illustrating an example of characteristics of thesemiconductor light emitting device.

This figure is plotted based on the experimental data shown in FIG. 4.In FIG. 7, the horizontal axis indicates a peak wavelength λp, and thevertical axis indicates an Al composition ratio z in the AlGaN layer ML.

A white circular mark in FIG. 7 indicates the Al composition ratio z atwhich a high output power OP is obtained for the indicated peakwavelength λp among the Al composition ratios z of 0.09, 0.14, 0.18 and0.30. A black square mark indicates the Al composition ratio z at whichan output power OP is relatively low for the indicated peak wavelengthλp among the Al composition ratios z of 0.09, 0.14, 0.18 and 0.30.

As can be seen from the example shown in FIG. 7, in the case where theAl composition ratio z is 0.30 (semiconductor light emitting device111), the output power OP is high when the peak wavelength λp is longerthan 515 nm. However, the output power OP is low when the peakwavelength λp is 500 nm or shorter.

In the region where the peak wavelength λp is longer than 515 nm, whenthe Al composition ratio z is 0.18 or lower, a high output power OPcannot be achieved.

On the other hand, in the relationship between the peak wavelength λpand an optical output, it is known that emission of light can beobtained at the highest efficiency in the range of 400 nm to 420 nm. Inview of this, for example, in the nitride semiconductor light source(for Blu-ray Disc etc., for example) having a similar configuration tothat of the fourth reference example without using the AlGaN layer ML,the wavelength of 405 nm, which provides light emission at highefficiency, is used. Based on this, it can be assumed that anappropriate Al composition ratio z when peak wavelength λp is 405 nm iszero. This condition is indicated by the point P0 in FIG. 7.

On the other hand, the condition under which a high output power OP canbe obtained obviously when the Al composition ratio z is 0.3 isindicated by the point Q0 in FIG. 7.

It is considered that the effect of providing the AlGaN layer ML isexhibited under the condition shown in the region around the line(center condition line L0) connecting the above point P0 and the pointQ0. The Center condition line L0 is z=0.0024λp−0.972. Here, z is an Alcomposition ratio in group III elements in the AlGaN layer ML. The λp(nanometer) indicates a peak wavelength of light emitted from the lightemitting part 40.

Further, as shown in FIG. 7, a high output power OP is obtained underthe conditions (conditions indicated by the region enclosed by a firstboundary condition line L1 and a second boundary condition line L2)around the center condition line L0. The first boundary condition lineL1 is defined by 1.15z=0.0024λp−0.972. The second boundary conditionline L2 is defined by 0.90 z=0.0024λp−0.972.

Therefore, a high output power OP can be achieved when the Alcomposition ratio z and the peak wavelength λp satisfy the followingcondition:1.15z>0.0024λp−0.972>0.90z.

In the first light emitting layer ELL a high output power OP can beachieved when the Al composition ratio z1 of the first AlGaN layer ML1and the peak wavelength λp satisfy the following condition:1.15z1>0.0024λp−0.972>0.90z1.

When the above condition is satisfied, a high luminance is achieved inthe wavelength region of 515 nm or shorter. Further, also when the Alcomposition ratio z in the AlGaN layer ML is 0.25 or smaller, a highluminance is obtained.

In the embodiment, the Al composition ratio z can be measured by meansof, for example, an Energy Dispersive X-ray Spectrometry (EDX), etc.Additionally, a Secondary Ion-microprobe Mass Spectrometer (SIMS), or astructural analysis method by an omega-2theta scan using an X-raydiffraction (XRD).

The thickness of a crystalline layer such as the AlGaN layer can beobtained, for example, based on a transmission electron microscope imageof a cross-section of the crystalline layer.

Each semiconductor layer in the semiconductor light emitting deviceaccording the embodiment may be grown using a growth method such asMetal-Organic Chemical Vapor Deposition (MOCVD), Metal-Organic VaporPhase Epitaxy (MOVPE) or the like.

When forming each semiconductor layer, the following materials may beused for raw materials.

For a Ga raw material, for example, TMGa (trimethylgallium), TEGa(triethylgallium) or the like may be used. For an In raw material, forexample, TMIn (trimethylindium), TEIn (triethylindium) or the like maybe used. For an Al raw material, for example, TMAI (trimethylaluminum)or the like may be used. For an N raw material, for example, NH₃(ammonia), MMHy (monomethyl hydrazine), DMHy (dimethyl hydrazine) or thelike may be used. For a Si raw material, for example, SiH₄ (monosilane)or the like may be used. For an Mg raw material, for example, Cp₂Mg(bis(cyclopentadienyl)magnesium) or the like may be used.

According to the embodiment, a semiconductor light emitting device ofhigh efficiency is provided.

In the specification, “nitride semiconductor” includes all compositionsof semiconductors of the chemical formula B_(x)In_(y)Al_(z)Ga_(1-x-y-z)N(0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z≦1) for which each of the compositionalproportions x, y, and z are changed within the ranges. “Nitridesemiconductor” further includes group V elements other than N (nitrogen)in the chemical formula recited above, various elements added to controlvarious properties such as the conductivity type, etc., and variouselements included unintentionally.

In the specification of the application, “perpendicular” and “parallel”refer to not only strictly perpendicular and strictly parallel but alsoinclude, for example, the fluctuation due to manufacturing processes,etc. It is sufficient to be substantially perpendicular andsubstantially parallel.

Hereinabove, exemplary embodiments of the invention are described withreference to specific examples. However, the invention is not limited tothese specific examples. For example, one skilled in the art maysimilarly practice the invention by appropriately selecting specificconfigurations of components included in semiconductor light emittingdevices such as n-type semiconductor layers, p-type semiconductorlayers, light emitting parts, light emitting layers, well layers,barrier layers, AlGaN layers, and electrodes, included in light emittingapparatuses, etc., from known art. Such practice is included in thescope of the invention to the extent that similar effects thereto areobtained.

Further, any two or more components of the specific examples may becombined within the extent of technical feasibility and are included inthe scope of the invention to the extent that the purport of theinvention is included.

Moreover, all semiconductor light emitting devices practicable by anappropriate design modification by one skilled in the art based on thesemiconductor light emitting devices described above as embodiments ofthe invention also are within the scope of the invention to the extentthat the purport of the embodiments of the invention is included.

Various other variations and modifications can be conceived by thoseskilled in the art within the spirit of the invention, and it isunderstood that such variations and modifications are also encompassedwithin the scope of the invention.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the invention.

What is claimed is:
 1. A semiconductor light emitting device,comprising: an n-type semiconductor layer including a nitridesemiconductor; a p-type semiconductor layer including a nitridesemiconductor; and a light emitting part provided between the n-typesemiconductor layer and the p-type semiconductor layer and including ann-side barrier layer and a first light emitting layer, the first lightemitting layer including: a first barrier layer provided between then-side barrier layer and the p-type semiconductor layer; a first welllayer contacting the n-side barrier layer between the n-side barrierlayer and the first barrier layer; and a first AlGaN layer providedbetween the first well layer and the first barrier layer, the firstAlGaN layer having a layered-form and including Al_(z1)Ga_(1−z1)N(0.25<z1≦1), a peak wavelength λp of light emitted from the lightemitting part being longer than 515 nanometers.
 2. The device accordingto claim 1, wherein a thickness of the first AlGaN layer has an RMSvalue of 0.5 nanometers or less.
 3. The device according to claim 1,wherein a major surface of the n-type semiconductor layer is a c-face.4. The device according to claim 1, wherein the first light emittinglayer further includes a first cap layer contacting the first AlGaNlayer and the first barrier layer and including a nitride semiconductor.5. The device according to claim 1, wherein in the first AlGaN layer, anarea of recessed regions or regions having through holes formed occupiesless than 10% of a layer surface of the first AlGaN layer.
 6. The deviceaccording to claim 1, wherein the first barrier layer includesIn_(x1)Ga_(1−x1)N(0≦x1<1), and the n-side barrier layer includesIn_(x2)Ga_(1−x2)N(0≦x2<1).
 7. The device according to claim 1, whereinthe light emitting part further includes a second light emitting layer,the second light emitting layer including: a second barrier layerprovided between the first barrier layer and the p-type semiconductorlayer; a second well layer contacting the first barrier layer betweenthe first barrier layer and the second barrier layer; and a second AlGaNlayer provided between the second well layer and the second barrierlayer, the second AlGaN layer having a layered-form and includingAl_(z2)Ga_(1−z2)N (0.25<z2≦1).
 8. The device according to claim 1,wherein a thickness of the first well layer is 1.0 nanometer or more and5.0 nanometers or less.
 9. The device according to claim 1, wherein athickness of the first barrier layer is 3 nanometers or more and 50nanometers or less.
 10. The device according to claim 1, wherein athickness of the n-side barrier layer is 3 nanometers or more and 20nanometers or less.
 11. A semiconductor light emitting device,comprising: an n-type semiconductor layer including a nitridesemiconductor; a p-type semiconductor layer including a nitridesemiconductor; and a light emitting part provided between the n-typesemiconductor layer and the p-type semiconductor layer and including ann-side barrier layer and a first light emitting layer, the first lightemitting layer including: a first barrier layer provided between then-side barrier layer and the p-type semiconductor layer; a first welllayer contacting the n-side barrier layer between the n-side barrierlayer and the first barrier layer; and a first AlGaN layer providedbetween the first well layer and the first barrier layer, the firstAlGaN layer having a layered-form and including Al_(z1)Ga_(1−z1)N(0<z1≦1), an Al composition ratio z1 in group III of the first AlGaNlayer and a peak wavelength λp (nanometer) of light emitted from thelight emitting part satisfying a condition of1.15z1>0.0024λp−0.972>0.90z1.
 12. The device according to claim 11,wherein a thickness of the first AlGaN layer has an RMS value of 0.5nanometers or less.
 13. The device according to claim 11, wherein amajor surface of the n-type semiconductor layer is a c-face.
 14. Thedevice according to claim 11, wherein the first light emitting layerfurther includes a first cap layer contacting the first AlGaN layer andthe first barrier layer and including a nitride semiconductor.
 15. Thedevice according to claim 11, wherein in the first AlGaN layer, an areaof recessed regions or regions having through holes formed occupies lessthan 10% of a layer surface of the first AlGaN layer.
 16. The deviceaccording to claim 11, wherein the first barrier layer includesIn_(x1)Ga_(1−x1)N(0≦x1<1), and the n-side barrier layer includesIn_(x2)Ga_(1−x2)N(0≦x2<1).
 17. The device according to claim 11, whereinthe light emitting part further includes a second light emitting layer,the second light emitting layer including: a second barrier layerprovided between the first barrier layer and the p-type semiconductorlayer; a second well layer contacting the first barrier layer betweenthe first barrier layer and the second barrier layer; and a second AlGaNlayer provided between the second well layer and the second barrierlayer, the second AlGaN layer having a layered-form and includingAl_(z2)Ga_(1−z2)N (0.25<z2≦1).
 18. The device according to claim 11,wherein a thickness of the first well layer is 1.0 nanometer or more and5.0 nanometers or less.
 19. The device according to claim 18, wherein athickness of the first barrier layer is 3 nanometers or more and 50nanometers or less.
 20. The device according to claim 19, wherein athickness of the n-side barrier layer is 3 nanometers or more and 20nanometers or less.